Certainly, atemi is an essential part of aikido. Apart from the use for distraction and such, I see atemi as an alternative route - showing the link between aikido and other budo.
Many aikido techniques are very similar to striking techniques in other martial arts - they're just done in a more rounded way, and in another tempo. Atemi shows this relation.
Nishio sensei used atemi also to show an attacker that he was mistaken in the attack, and therefore to give him a chance to reconsider. With the irimi step and the immediate atemi, the battle is already over, so to speak.
The aikido technique that follows the atemi is the nice solution, as opposed to really hitting the attacker.
Nishio sensei had the same idea in his sword art, where it was even more obvious.
For atemi to work in the way mentioned above, or as a distraction, and of course if used as an actual strike, it needs to be trained. A distraction has to be convincing, a strike has to be powerful. That takes time to learn - and aikidoists have so much on the curriculum that they rarely find that time.
I have seen a lot of sloppy atemi.
For learning atemi, I think it is a good idea to examine how the striking arts - for example karatedo - do it. They should know.
On the other hand, I don't want to train aikido in such a way that atemi becomes necessary, in order to complete the techniques. I think it is more interesting to try and find a way of doing the aikido techniques, which works even without atemi.
When we rely too much on atemi, our aikido risks to lose its identity, and become kind of boxing that just ends with a push or a pinning. There is also a risk that we do not work enough on perfecting the technique, because we think that the atemi itself "took care of business".
Then again, even if we try to pursue an aikido where atemi is not necessary, it is good to have
It enriches one's aikido, and leads to a deepeer understanding of the martial arts.